The present invention is related to temperature measurement, and more particularly to temperature measurements using a transistor or diode as a sensor.
Temperature measurement using a transistor as a sensor is a common application in the semiconductor area. Such a temperature measurement is done by applying two different currents to the transistor each resulting in a respective base-emitter voltage. The difference between the two different base-emitter voltages is proportional to the absolute temperature of transistor 144. The following equation defines the relationship between the difference between base-emitter voltage measurements and absolute temperature:ΔVbe=Vbe2−Vbe1=n*kT/q*1n(I2/I1).The ‘n’ term is known as the non-ideality factor or emission coefficient is assumed to be a constant (n=1.008) for diodes and transistors.
An example of such a temperature measurement circuit 100 is shown in FIG. 1a. Turning to FIG. 1a, temperature measurement circuit 100 includes a transistor 120 that is used as a temperature sensor. The collector and the base of transistor 120 are electrically coupled to a variable current source 110. Further, the base of transistor 120 is electrically coupled to one input of an analog to digital converter 130, and the emitter of transistor 120 is electrically coupled to another input of analog to digital converter 130. Analog to digital converter 130 is operable to receive the voltages at the base and emitter of transistor 120, and to provide a ΔVbe output 135 representing the difference between two different base to emitter voltages. ΔVbe output 135 is provided to a temperature calculation circuit 140 that provides an uncorrected temperature output 145.
In some cases, an input filter 134 including a series resistor 131, a series resistor 132, a and a capacitor 133 is used. Input filter 134 is operable to filter noise from the voltages received from the base and emitter of transistor 120. While input filter 134 operates to increase the accuracy ΔVbe output 135 and thereby increase the accuracy of uncorrected temperature 145, the series resistance introduced by input filter 134 results in an error in uncorrected temperature 145. In particular, the resistance introduced by series resistor 131 and series resistor 132 (and in some cases non-idealities of transistor 120) causes a voltage drop that is a function of the magnitude of an applied current. This voltage drop is described by the following equation:ΔVbe=Vbe2−Vbe1=(Ie2−Ie1)*Rs+n*kT/q*ln(Ic2/Ic1).Ie1 is the current passing through the emitter upon application of a first current, and Ic1 is the current passing through the collector upon application of the same current. Ie2 and Ic2 are similarly emitter and collector currents corresponding to the application of a second current. Rs is the series resistance. The voltage drop described by the aforementioned equation will create a temperature measurement error if not taken into account by the circuit.
To correct for the aforementioned temperature error, some circuits have included a backend offset circuit designed to add or subtract a calculated constant from uncorrected temperature 145 and thereby achieve a corrected temperature. FIG. 1b shows an example of one such temperature calculation circuit 101. As shown, temperature calculation circuit 101 is substantially similar to temperature calculation circuit 100, except for the addition of a temperature offset adder circuit 150. Temperature offset adder circuit 150 receives uncorrected temperature 145 and a programmed temperature offset input 147. The two inputs are added together to create a corrected temperature output 155. While such an offset approach can effectively correct calculation errors at a given point on an operational curve, the inaccuracy of the calculated temperature still exists as operation moves farther from the aforementioned offset corrected point on the operational curve.
Thus, for at least the aforementioned reasons, there exists a need in the art for advanced systems and devices for temperature measurement.